Seasonal Variation in Subscapularis Tendon Strain in Baseball Pitchers Assessed by Ultrasonography-based Analysis

Abstract Background: Repetitive loading injuries of the rotator cuff tendons are common in baseball pitchers. Ultrasonography-based strain analysis shows promise for in vivo monitoring of cumulative tendon loading but faces technical challenges in the rotator cuff. We developed a novel ultrasonography-based strain analysis procedure to test the hypothesis that regular-season pitching increases the peak strain of the subscapularis tendon. Methods: Ultrasound image sequences of subscapularis tendon were obtained from the dominant shoulders of nine pitchers during isotonic motion in the preseason and regular season. Tendon displacement during the eccentric phase was estimated using optical flow tracking, validated against NCORR, a reliable software for quantifying material deformation. Results: Peak strain was significantly higher in the regular season (18.7 ± 3.1%) compared to the preseason (8.8 ± 2.7%), suggesting an elevated risk of tendon microdamage. Conclusion: Our findings indicate that intense pitching during the season may elevate tendon strain, highlighting the potential of strain analysis for monitoring tendon loading-related changes. The proposed procedure is robust, reliable, and easy to implement, offering a practical tool for tracking in-season tendon loading in pitchers.